A power converter is a power processing circuit that converts an input voltage waveform into a specified output voltage waveform. In many applications requiring a DC output, switched-mode DC-DC converters are frequently employed to advantage. DC-DC converters generally include an inverter, a transformer having a primary winding coupled to the inverter and a rectifier coupled to a secondary winding of the transformer. The inverter generally includes a switching device that converts the DC input voltage to an AC voltage. The transformer then transforms the AC voltage to another value and the rectifier generates the desired DC voltage at the output of the DC-DC converter.
Conventionally, the switching device used in the inverter is a controllable switch such as a metal-oxide semiconductor field-effect transistor (MOSFET). The controllable switch in the inverter is modulated by periodically being driven into conduction and non-conduction states to maintain a required output voltage for the power converter. The rectifier may include passive rectifying devices that conduct the load current only when forward-biased in response to the input waveform to the rectifier. Passive rectifying devices, however, generally cannot achieve forward voltage drops that are low enough to provide a desired conversion efficiency of the DC-DC converter. To achieve a higher level of efficiency, DC-DC converters may therefore use synchronous rectifiers.
A synchronous rectifier replaces the passive rectifying devices of the conventional rectifier with a controllable switch. This controllable switch is also periodically driven into conduction and non-conduction states in synchronism with the periodic waveform of the AC voltage. The rectifier switches typically exhibit resistive-conductive properties and may thereby avoid the higher forward voltage drops inherent in the passive rectifying devices.
A metal-semiconductor field-effect transistor (MESFET) may be used as a controllable switch. The MESFET consists of a conducting channel positioned between a source and drain contact region. A carrier flowing from the source to the drain is controlled by a Schottky metal gate. Control of the channel is accomplished by varying the depletion layer width underneath the metal contact which modulates the thickness of the conducting channel and thereby the current.
A key advantage of the MESFET is the higher mobility of the carriers in the channel as compared to a MOSFET. The higher mobility leads to a higher current, transconductance and transit frequency for the device. A higher transit frequency makes the MESFET of particular interest for higher frequency applications. The use of a Gallium-Arsenide metal-semiconductor field-effect transistor (GaAsMESFET) rather than a Silicon MESFET provides additional advantages in that the room temperature mobility is more than five times larger and the saturation velocity is about twice that of Silicon. These qualities make the GaAsMESFET particularly attractive for use as a switching device in high speed applications requiring low losses. For a better understanding of Gallium-Arsenide devices see "Optimum Silicon and GaAs Power Field-Effect Transistors for Advanced High-Density, High Frequency Power Supply Applications," by K. Shenai, C. Korman, and B. Baliga, HFPC 1989, and "10 MHz PWM Converters With GaAs VFETs", by R. Kollman, G. Collins, and D. Plumton, APEC 1996, both of which are incorporated herein by reference.
Unlike the MOSFET in switching applications, however, the MESFET structure contains the Schottky metal gate. The Schottky metal gate limits the forward bias voltage on the gate to the turn-on voltage of a Schottky diode, which may be about 0.7 volts for Gallium-Arsenide. Therefore, the gate of a MESFET responds as a forward biased diode when the MESFET is used as a switch and in its conducting state. Additionally, the MESFET requires a bias voltage of an appropriate polarity to force it into a non-conducting state, since the MESFET conducts for a gate-to-source voltage of zero volts. As a result of these characteristics, it is difficult to use the MESFET as a controllable switch in many power converters since driver circuits and sources of appropriate bias supply voltages are often complex and more difficult to construct.
Accordingly, what is needed in the art is a driver for a MESFET that resolves the deficiencies and reduces the complexity associated with the prior art driver circuits.